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Abstract:

Laser drilling is a technologically important process, used in a variety of material processing applications. In many cases, the precise size and shape of individual holes, as well as their location, are of considerable importance. The current work has concentrated on the details of the physical phenomena which take place during laser drilling and on formulating models for quantitative prediction of the effects of material properties and laser drilling conditions on the size and shape of the hole and of the surrounding heat-affected zone. Drilling various materials, including metals, composites and coated specimens, has been undertaken, using Nd:YAG and CO2 lasers with power densities ranging from 109 to 1011 W m-2. Experimental results obtained recently by other workers have also been examined in some detail. One of the issues of interest is the relative significance of the removal of material during drilling by vaporisation and by melt ejection. Melt ejection is more energy-efficient, but it tends to lead to irregularly-shaped holes and also causes more contamination of the surroundings. When drilling metallic materials, the level of melt ejection has been found to depend, not only on the thermophysical properties of the specimen, but also on the laser beam parameters. For a given pulse power, a shorter pulse width produces more concentrated heat fluxes, which promotes vaporisation. Improvements in hole quality are observed when multiple pulses are used. Multiple-pulse operation, in the form of a series of short intense pulses, allows vapour to escape before the pressure can build up and promote melt ejection. Little or no melt ejection occurs with carbon fibre reinforced polymer composite specimens, presumably because the fibres do not melt and the viscosity of the molten matrix is very high. Carbon fibres in the heat-affected zone around the hole were observed to have become swollen by up to about 50% in diameter. This is attributed to irreversible changes in the arrangements of the basal planes in the turbostratic structure, caused by rapid thermal expansion. This effect may have been accentuated by the pressurisation of fine pores within the structure of the fibres.